In general, a refrigerator is constituted by at least a compressor, a condenser, an expansion mechanism (such as an expansion valve), and an evaporator, or furthermore by a dryer, and has a structure that a mixture of a refrigerant and a refrigerator lubricating oil is circulated in the closed system. As the refrigerant for a refrigerator, a chlorine-containing compound, such as a chlorofluorocarbon, has been widely used, but in consideration of the environment protection, it is being replaced by a compound containing no chlorine, such as a hydrofluorocarbon (HFC), and a natural refrigerant, such as carbon dioxide. Examples of the hydrofluorocarbon in practical use include a saturated hydrofluorocarbon (which may be hereinafter referred to as a saturated HFC), which is represented by 1,1,1,2-tetrafluoroethane, difluoromethane, pentafluoroethane, and 1,1,1-trifluoroethane (which may be hereinafter referred to as R134a, R32, R125, and R143a, respectively).
However, as a refrigerant for a car air-conditioner, it is currently difficult to use carbon dioxide requiring high pressure as the major component, and the saturated HFC also has a problem of the high global warming potential. Under the circumstances, it is being considered in recent years to use an unsaturated fluorinated hydrocarbon compound, such as HFO1234ze and HFO1234yf, as a refrigerant capable of being used in a car air-conditioner, irrespective of the low global warming potential thereof.
For a refrigerant containing an unsaturated fluorinated hydrocarbon compound, it is considered to use an oxygen-containing compound, such as a polyoxyalkylene glycol compound, a polyol ester compound, and a polycarbonate compound, as a base oil of a refrigerator lubricating oil.
As the polyol ester-based refrigerator lubricating oil used for a refrigerant containing an unsaturated fluorinated hydrocarbon compound, it has been known that, for example, an ester of hindered alcohol, such as pentaerythritol, is contained as a base oil, and an antioxidant, an extreme pressure agent, an anti-wear agent, an oxygen scavenger, and the like are blended as additives depending on necessity (see, for example, PTLs 1 and 2). The examples of the extreme pressure agent described therein include a sulfurized olefin, a sulfurized oil or fat, and the like, and the examples of the anti-wear agent and the oxygen scavenger described therein include various monosulfide compounds.